In a typical radio communications network, wireless terminals, also known as mobile stations and/or user equipments (UEs), communicate via a Radio Access Network (RAN) to one or more core networks. The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB (NB)” or “eNodeB (eNB)”. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site or an antenna site in case the antenna and the radio base station are not collocated. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. The base stations communicate over the air interface operating on radio frequencies with the wireless terminals within range of the base stations. Transmissions from the wireless terminals to the radio base station are defined as uplink (UL) transmissions and transmissions from the radio base station to the wireless terminal is defined as downlink (DL) transmissions.
In some versions of the RAN, several base stations are typically connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
A Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS Terrestrial Radio Access Network (UTRAN) is essentially a RAN using Wideband Code Division Multiple Access (VVCDMA) and/or High Speed Packet Access (HSPA) for user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for e.g. third generation networks and further generations, and investigate enhanced data rate and radio capacity.
Specifications for the Evolved Packet System (EPS) have been completed within the 3GPP and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base stations are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the radio base stations, e.g., eNodeBs in LTE, and the core network. As such, the RAN of an EPS system has an essentially “flat” architecture comprising radio base stations without reporting to RNCs.
In heterogeneous networks, the radio base stations or eNBs have different DL output power, e.g., macro eNBs with high output power and pico eNBs with low output power. “Macro eNBs” meaning radio base stations providing radio coverage over a macro cell and “pico eNBs” meaning radio base stations providing radio coverage over a pico cell. This imbalance in the transmission power combined with the conventional cell selection mechanism leads to two problems.
In LTE, a Reference Signal Received Power-based (RSRP-based) cell selection scheme is often used. In this cell selection, wireless terminals are associated with the cell from which the strongest DL power is received. As the macro eNB has higher output power than the pico eNB, wireless terminals are more likely to connect to the macro cell or macro eNB. The pico cell size is thus relatively small compared to the macro cell size, which may result in low wireless terminal uptake and small macro offloading by the pico cell. In addition to that, with the RSRP-based cell selection scheme, some of the macro connected wireless terminals experience a lower path loss to the pico eNB, and thus are not connected to the best cell from an UL perspective.
To increase offloading of the macro cell by the pico cells and to improve UL performance, there is a need to increase the size of the pico cells. This can be done with Cell Range Expansion (CRE) based cell selection, where a Cell Selection Offset (CSO) is added to the RSRP of the pico eNB before comparison, see FIG. 1. FIG. 1 discloses UL/DL power imbalance and a cell range expansion. A macro base station comprises two transmitters of 20 W each and a pico base station comprises two transmitters of 0.5 W each. In FIG. 1 a cell border for DL is moved by CRE using a CSO. The UL border is kept as before. With CRE, a wireless terminal may be connected to a pico cell even though the received DL power from the macro cell is stronger. In case of inter-frequency deployment, a large CSO is conceivable for the DL but in case of intra-frequency deployment, applying a CSO introduces the additional challenge of strong DL interference.
There is an interference challenge in intra-frequency deployment; when the macro and pico layers are operated on the same frequency, pico wireless terminals, or wireless terminals connected to the pico, in the CRE region experience negative DL Signal to Interference plus Noise Ratio (SINR) due to strong DL interference caused by the macro cell. So, from a DL perspective, the CSO must be chosen considering the trade-off between signal quality reduction for wireless terminals in the CRE region and traffic offload improvement. It is often the case that a small to moderate value for the CSO is optimal for the DL. By contrast, from an UL perspective, a large CSO results in a better UL signal quality and a larger traffic offload by the pico layer. So, there is a mismatch between the optimal CSO for DL and UL in intra-frequency deployments.
To support large CSO values in the DL, time domain Inter-cell interference coordination (ICIC) and advanced wireless terminal receivers are supported in LTE release 10 and release 11 respectively. The idea is to protect the DL signals for pico wireless terminals in CRE region by applying almost blank subframes in the macro eNB and interference cancellation of Cell—specific Reference Signals (CRS) in the pico wireless terminals. However, these methods are supported for a CSO up to 9 dB only. This may not be sufficient to achieve the optimal cell border for the UL, which is when the CSO compensates for the transmit power imbalance between macro and pico eNBs. In addition, time domain ICIC has a negative impact on the capacity of the macro layer due to the reduction of schedulable subframes. So, these solutions have limitations to the supported CSO and may reduce the performance of the radio communications network.